Technical Field
The present disclosure is directed to semiconductor devices and, in particular, semiconductor devices having a reduced middle of the line resistance.
Description of the Related Art
Modern integrated circuits often include a large number of transistors, generally field effect transistors, which are interconnected with other transistors or devices. These transistors generally include interconnect contacts for the drain, source and gate of the transistor. Wiring is connected to the interconnect contacts of individual transistors in order to connect the transistors to other transistors and devices.
Integrated circuits are fabricated in several stages. In the first stage, referred to as the front end of the line (FEOL), individual devices such as transistors are patterned in the semiconductor. While the transistors and other devices are patterned in the FEOL stage, connection contacts to the gate, drain and source of the individual devices are created in a middle of the line (MOL) stage. The back end of the line (BEOL) comes after the transistors are formed, and is when the individual devices are connected to one another with wiring on the semiconductor wafer.
In the MOL stage, connectors to the gate, drain and source of transistors are formed. The MOL stage involves adding a trench silicide layer (TS), generally made of tungsten, to the source and drain, and forming a tungsten contact layer (CA) over the TS. The TS and CA layers both require the use of a liner, such as a TiN liner. The tungsten contact, CA, in addition to the TiN liner adds significant resistance to the connections formed in the MOL stage because tungsten has a low conductivity compared to other metals such as copper.
The gate of the transistor undergoes a similar process during the MOL stage, having a second tungsten contact layer (CB) formed over the gate conductor, with a liner material such as TiN between the gate conductor and CB. The tungsten contact layer, CB, and the related TiN liner also add significant resistance to the gate conductor contact in the MOL stage. Tungsten is used for the contacts CA and CB instead of the more conductive material copper in order to remove the dangers of diffusion when copper is deposited close to the gate, drain and source. However, the use of tungsten also adds significantly higher resistance to the contacts because tungsten has a much lower conductivity than copper.
As integrated circuits continue to scale down in size from 10 nm to 7 nm, reducing the resistance of the connections created in the MOL step is becoming an important part of semiconductor fabrication.